1. Field of the Invention
This invention relates to a method of connecting a metal ring gear to a boss portion made of metal.
2. Description of the Prior Art boss portion made of metal, e.g., for connecting a ring gear of a differential of an automobile to a differential case, there has been known a method in which the former is mechanically connected to the latter by plastic deformation of the metal material such as plastic flow connection, riveting or the like. For example, there is disclosed, in Japanese Unexamined Patent Publication No. 61(1986)-42445, a method of connecting a ring gear to a differential by plastic flow connection.
When the ring gear is connected to the boss portion by such a method, the parts associated with the ring gear can be reduced in weight, the parts can be manufactured at a lower cost and the manhour required for the assembly of the ring gear can be reduced as compared with a case where the ring gear is connected to the boss portion by use of bolts and nuts.
When the ring gear is manufactured, at least the tooth portion thereof is heat-treated in order to increase the strength and the resistance to wear.
By subjecting the ring gear to surface hardening heat treating such as carburizing or nitriding treatment, residual compression stress is produced in the hardened surface portion, whereby fatigue properties to repeated loading is improved as well as resistance to wear.
Further there has been known the fact that, by shot peening, scale on the surface of the member can be removed and the surface profile can be improved and at the same time, residual compression surface can be produced in the surface portion of the member.
However the method in which the ring gear is mechanically connected to the boss portion by plastic deformation of the metal material gives rise to a problem that an internal pressure acts on the ring gear in response to plastic deformation of the metal material and a tensile stress acts on the root of the tooth portion in the circumferential direction of the gear, which adversely affects fatigue life of the gear.
That is, even in the ring gear which has been subjected to the surface hardening heat treating, the tensile stress which acts on the root of the tooth portion as a result of connection of the ring gear to the boss portion reduces the residual compression surface in the surface portion and accordingly a desired fatigue life cannot be stably obtained.
Further the method of connecting the ring gear to the boss portion can produce deformation in the tooth portion of the ring gear and deteriorate precision of the gear, which adversely affects mesh of the gears and causes noise.